Various polycondensation polymers represented by a polyester resin such as polyethylene terephthalate (hereinafter abbreviated as “PET”) have been noticed as having characteristics such as superior heat resistance and mechanical properties and also recently as environmentally friendly recyclable material and has widely been used as fibers, magnetic tapes, packaging films, sheets or injection molded products in various applications or preform for producing beverage containers.
A hollow product produced by blow molding of preform, in particular, has superior characteristics of such as light weight, impact resistance and transparency, along with recyclability, by which demand as containers for various beverage such as carbonated drink or juice, tea and mineral water, or liquid seasoning such as soy sauce, sauce and salad oil, cosmetic and liquid detergent has rapidly increased and such market will be expected to expand further in the future.
These containers require not only to have superior strength, impact resistance and transparency but also not to affect taste of contents. Therefore, as a polycondensation polymer used, high degree of polymerization, no coloring and high quality with low content of impurity generated by thermal decomposition such as acetaldehyde, are required. Furthermore, such a polycondensation polymer is strongly required to be produced industrially stably, in good productivity and in low cost.
In case of a hollow product made of a PET resin, preform is produced by injection molding of resin pellets prepared by solid phase polymerization yielding low coloring and small acetaldehyde content, however, recently as a low cost production method, such a method has been proposed for producing preform by transferring a PET resin produced by continuously melt polymerization into a preform molding machine in molten state as it is (see, for example, patent documents 1 to 16).
To produce a molded product with stable quality, it is necessary to adjust quality and amount of a molten resin transferred to a molding machine at constant level. However, methods proposed up to now are not easy to stably maintain quality of a molded product, because a polycondensation resin represented by a PET resin causes a thermal decomposition reaction in a long period of residence as molten state, which results in lower quality problems such as, depending on residence time, accumulation of a decomposed product such as acetaldehyde, decrease in molecular weight and coloring. In addition to the above, molecular weight may decrease during transferring a resin discharged from a polymerization reactor to a molding machine, which may result in variation of melt viscosity and thus transferring amount. The transferring amount to the molding machine may also be changed due to variation of melt viscosity of a resin discharged from the polymerization reactor by change in polymerization system before that.
Methods conventionally proposed have also difficulty in matching “resin producing rate” in a melt polymerization reactor and “resin molding rate” in a molding machine. For example, when a molding machine breaks down or a molding machine is stopped due to cleaning of an oligomer, and the like adhered to a mold, “resin molding rate” in a molding machine is below “resin producing rate” in a melt polymerization reactor. On the contrary, “resin producing rate” in the melt polymerization reactor may be below “resin molding rate” in the molding machine when the reaction is stalled in a polymerization system. As described above, mutual variation of “resin producing rate” and “resin molding rate” results in variation of resin quality due to change in melt resin residence time at the upper stream than the molding machine.
Furthermore, an injection molding machine such as a preform molding machine has a problem that flow of a molten resin received from the melt polymerization reactor is intermittent due to repeated molding of resin metering and injection cycle which always causes variation of melt resin residence time.
Patent document 2 discloses a compensation method for the intermittent flow in an injection molding machine by opening a feed port of a molding machine at specified sequence to substantially make material flow constant during molding using multiple molding machines in practicing a method for producing the above-described molded product, however, this patent document does not envision variation of quality or melt viscosity of a resin fed to a molding machine. As described above, mutual variation of “resin producing rate” and “resin molding rate” results in variation of resin quality. This method also has a problem of generation of a large quantity of defective products or loss till molded product quality is stably recovered once failure generates, because complicated exchange system or sequence is required.
Patent documents 4, 6, 7, 9, 10 and 14 disclose a method for making aldehyde harmless, which is generated and accumulated in a resin during polymer melt residence in melt polymerization and till cooling and solidification after melt polymerization and transferring to a molding machine, by the addition of an acetaldehyde scavenger. However, it is not effective to a problem of decreasing in resin molecular weight, although useful as measures for acetaldehyde in case of variation of resin quality and transferring amount of a resin fed to a molding machine and also mutual variation of “resin producing rate” and “resin molding rate”. Furthermore, there is also a problem of worsening resin hue by an acetaldehyde scavenger itself.
Even other patent documents among 1 to 16 have not proposed technology at all to eliminate variation of molded product quality in case of variation of quality and transferring amount of a resin fed to a molding machine and also mutual variation of “resin producing rate” and “resin molding rate”.    [Patent Document 1] JP No. 3,345,250    [Patent Document 2] JP-A-11-508836    [Patent Document 3] JP-A-11-511187    [Patent Document 4] U.S. Pat. No. 5,656,221    [Patent Document 5] JP-A-2000-506199    [Patent Document 6] JP-A-2002-514239    [Patent Document 7] JP-A-2001-516297    [Patent Document 8] WO 98/41559    [Patent Document 9] JP-A-2001-516389    [Patent Document 10] JP-A-2001-517164    [Patent Document 11] JP-A-2000-117819    [Patent Document 12] WO 2004/24577    [Patent Document 13] US-A-2005-29712    [Patent Document 14] DE-A-10356298    [Patent Document 15] JP-A-2005-171081    [Patent Document 16] JP-A-2005-193379